1. Field of the Invention
The present invention relates, in general, to high intensity discharge lamps. In particular, the invention relates to a metal halide lamp in which a rare earth halide is sealed in the arc tube, as a luminous metal.
2. Description of the Related Art
A high intensity discharge lamp, i.e., a so-called HID lamp, has been used as an outdoor lighting, etc. However, in recent years, the high intensity discharge lamp is further used as a light source of projectors or as an indoor lighting of stores. The above-described extended use of the high intensity discharge lamp is promoted by the miniaturization of the lamp.
In a conventional high intensity discharge lamp, opposite ends of an arc tube thereof are pinched to form respective bases (a double base type) in which a pair of electrodes are supported. Thus, the external size of the arc tube is large, as compared with the discharge space of the arc tube. Heat discharge from the bases of the arc tube also is large, resulting in the increase in heat loss of the lamp. To solve the above-described problems, a single base type arc tube is employed in the conventional high intensity discharge lamp. A pair of electrodes are supported in the single base. Thus, heat loss of the arc tube including the single base is reduced, and the external size thereof also is reduced, as compared with the arc tube which has two bases.
A high intensity discharge lamp may be a mercury lamp, a metal halide lamp, or a high pressure sodium lamp. The conventional high intensity discharge lamp has a high efficiency, and a high light output. The conventional high intensity discharge lamp also has a high brightness, and a high color rendering property. In particular, a metal halide lamp has a high efficiency and a high color rendering property, as compared with other types of the conventional high intensity discharge lamp. Furthermore, a metal halide lamp in which a rare earth halide, e.g., dysprosium (Dy), is sealed in the arc tube thereof as a luminous substance has a higher efficiency and a higher color rendering property, as compared with a metal halide lamp in which other metal halide, e.g., sodium (Na), is sealed in the arc tube thereof. However, in the above-described metal halide lamp in which the rare earth halide is sealed in the arc tube thereof, an operation life thereof is relatively insufficient. Thus, it is undesirable to use the above-described metal halide lamp as a light source of ,e.g., projector, because of the insufficient operation life. The wall loading of the above-described metal halide lamp is designed at a high level, e.g., 20.about.70 w/cm.sup.2, to achieve a high output in spite of a small external size thereof. Thus, the temperature of the arc tube made of quartz glass tends to increase. In addition, an excess amount of the rare earth halide is sealed in the arc tube, as compared with an amount thereof which is needed during the operation. Thus, the excess amount of the rare earth halide, in particular dysprosium (Dy), reacts on the silicon element contained in the arc tube of a high temperature during the operation, and the arc tube tends to lose the transparency. If the loss of the transparency of the arc tube extremely proceeds, in the event, bursting of the arc tube may occur. Furthermore, the excess amount of the rare earth halide causes spattering of the electrode material, i.e., tungsten, during the operation. Thus, the arc tube will become black faster than usual, or the lamp voltage will increase, resulting in the decrease in lumen maintenance factor of the lamp. In particular, the lumen maintenance factor of the conventional metal halide lamp wherein dysprosium is sealed in the arc tube decreases to 50% after 500 hours operation.